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CEM 3 fault?

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Old Jan 10th, 2021, 17:05   #1
360beast
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Default CEM 3 fault?

I've been to see a customer today with a 480, I've never worked on one before so had to do a bit of research before going.

At first he thought it was completely fried as he tried to charge it using his electric Mercedes, then he charged the battery on the car using a normal battery charger. The ignition lights came on but the starter just clicked so I jump started it with my 940 and it ran fine.

However the drivers electric window wound itself down! But won't go back up and the passenger electric window is shut luckily but won't go down. The main beam is on even with the light switch in the off position and the best bit is I have checked all the fuses have power (all at 13V with engine running) but the 30A fuse for the main beam has no power either side of the fuse! So how on earth are they on? It could be I hadn't quite got the multimeter in the correct position but I managed to check every other fuse fine so I doubt it as I checked it several times.

My thinking is the CEM 3 module is dodgy, the car is an L reg 1.7 non turbo 480 ES.

Any advice appreciated
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Old Jan 11th, 2021, 13:22   #2
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All the functions you describe are certainly controlled by the CEM, so I think you are looking in the right direction.

I wonder if trying to charge it using an electric Mercedes has somehow fried parts of the CEM? I know nothing about electric cars and their systems, but a normal jump start such as you performed with your 940 certainly shouldn't have hurt the CEM at all.

I don't know any of the background to the problem he was having, and of course you might not either, but sometimes the connections into the CEM can be slightly dislodged by rummaging around in the glove-box/passenger-side footwell side area if by any chance that has happened.

I would be inclined to remove the plastic trim down there (pull the weather-strip clear, sharp tug to pull up the inner door-sill trim, sharp tug to pull off bonnet release knob, then one screw and pull the plastic panel away (it should be held on two clips if they are still present after all these years).

There are three multi-pin connections into a CEM3 to check are tightly home.

Otherwise, best bet is to try another CEM...

Hope that helps!

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Old Jan 11th, 2021, 15:53   #3
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Quote:
Originally Posted by 360beast View Post
but the 30A fuse for the main beam has no power either side of the fuse! So how on earth are they on? It could be I hadn't quite got the multimeter in the correct position but I managed to check every other fuse fine so I doubt it as I checked it several times.
The electronics are designed with failsafe modes.
If comms are lost, some or all devices go to a failsafe mode: this means for example headlights on.
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Old Jan 11th, 2021, 16:16   #4
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Thanks both for the advice, I have advised the customer of what you have both said. He is going to buy another CEM 3 (he needs one for one of his other 480s too) so if that doesn't fix it then atleast he has it for his other 480.

I'll update how I get on with it.
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